Internal Coordination of Plant Responses to Drought and Evaporational Demand
187
stress and hormonal signals, particularly abscisic acid (ABA). Under water
stress, ABA is produced or liberated from storage sites in leaves, roots, or
even the guard cells themselves, and transported to the site of its action, the
guard cell plasmalemma. There it changes the membrane permeability by
depolarization of the membrane potential, and enhances thereby the efflux
of potassium ions (summaries of recent findings about the mechanisms:
Losch 1989, 1993b). Altered guard cell solute contents finally induce changed pore apertures as a result of altered turgor balances between the guard
cells and the surrounding epidermis.
During the decade 1970-1980, wide knowledge was accumulated about
the effects of plant environmental factors on stomatal behavior, particularly
about the effects of air humidity (Lange et al. 1971; Hall et al. 1976; Losch
and Tenhunen 1981). During the 1980s, changes occurred in the appreciation of internal plant effects on stomatal regulation. Furthermore, the focus
was extended to the coordination of morphological structure and functional
processes within different plant parts. In this way, an integrated view of
whole-plant responses to the environment becomes apparent which may
allow an extension from whole-plant individuals to stands of vegetation.
9.3 Root-Leaf Signals Under Moisture Shortage Contribute
to Drought A voidance Responses of Leaves
Contradictory findings concerning a threshold value for stomatal closure
under the influence of decreasing water potential (Hsiao 1973; Biscoe et al.
1976; Davenport et al. 1977) and hysteresis effects of stomatal behavior
after relief of water stress ("after effects"; e.g., Fischer et al. 1970) provided
the first indications that leaf conductance does not depend simply on immediate turgor hydraulics and leaf water potentials. Schulze and Kiippers
(1979) found decreasing conductances of Corylus avellana leaves under
controlled conditions when plant water potentials were progressively lowered during a period of several days. In contrast, reduced water potentials
during the course of a day did not negatively affect leaf conductances if leaf
turgor values approached zero. Field measurements of barley leaf conductances during diurnal courses gave high values even at actual water potentials of less than -2 MPa and turgor values near zero (Losch et al. 1992).
Leaf conductances were reduced only during a prolonged drought when
water deficits of the sandy soil in the barley field increased corresponding to
a flag leaf predawn water potential of approximately -1.6 MPa. Decreased
maximal conductances with lower predawn water potentials at progression
of seasonal drought, and continuously high conductances irrespective of low
noon water potentials at sufficient soil water supply in plants from winter or
summer rain climates (Losch et al. 1982; Ullmann 1985) may similarly
indicate a stomatal response to long-term but not to short-term decreases
187
stress and hormonal signals, particularly abscisic acid (ABA). Under water
stress, ABA is produced or liberated from storage sites in leaves, roots, or
even the guard cells themselves, and transported to the site of its action, the
guard cell plasmalemma. There it changes the membrane permeability by
depolarization of the membrane potential, and enhances thereby the efflux
of potassium ions (summaries of recent findings about the mechanisms:
Losch 1989, 1993b). Altered guard cell solute contents finally induce changed pore apertures as a result of altered turgor balances between the guard
cells and the surrounding epidermis.
During the decade 1970-1980, wide knowledge was accumulated about
the effects of plant environmental factors on stomatal behavior, particularly
about the effects of air humidity (Lange et al. 1971; Hall et al. 1976; Losch
and Tenhunen 1981). During the 1980s, changes occurred in the appreciation of internal plant effects on stomatal regulation. Furthermore, the focus
was extended to the coordination of morphological structure and functional
processes within different plant parts. In this way, an integrated view of
whole-plant responses to the environment becomes apparent which may
allow an extension from whole-plant individuals to stands of vegetation.
9.3 Root-Leaf Signals Under Moisture Shortage Contribute
to Drought A voidance Responses of Leaves
Contradictory findings concerning a threshold value for stomatal closure
under the influence of decreasing water potential (Hsiao 1973; Biscoe et al.
1976; Davenport et al. 1977) and hysteresis effects of stomatal behavior
after relief of water stress ("after effects"; e.g., Fischer et al. 1970) provided
the first indications that leaf conductance does not depend simply on immediate turgor hydraulics and leaf water potentials. Schulze and Kiippers
(1979) found decreasing conductances of Corylus avellana leaves under
controlled conditions when plant water potentials were progressively lowered during a period of several days. In contrast, reduced water potentials
during the course of a day did not negatively affect leaf conductances if leaf
turgor values approached zero. Field measurements of barley leaf conductances during diurnal courses gave high values even at actual water potentials of less than -2 MPa and turgor values near zero (Losch et al. 1992).
Leaf conductances were reduced only during a prolonged drought when
water deficits of the sandy soil in the barley field increased corresponding to
a flag leaf predawn water potential of approximately -1.6 MPa. Decreased
maximal conductances with lower predawn water potentials at progression
of seasonal drought, and continuously high conductances irrespective of low
noon water potentials at sufficient soil water supply in plants from winter or
summer rain climates (Losch et al. 1982; Ullmann 1985) may similarly
indicate a stomatal response to long-term but not to short-term decreases